Preforms are tempered so that the part of the preform to be deformed in a mold blowing process is heated to ensure its deformability. Normally, the preform has a head area with a thread and collar that is not to be deformed and is heated moderately or not at all. Furthermore, the preform normally has a finger-shaped area that is to be deformed in the mold blowing process. The phrase “tempering of preforms” refers in the following to the tempering of areas that are to be deformed, i.e., usually in the finger-shaped area.
Selectively tempering preforms serves to deform various areas in the preform that have different temperatures. In the deforming process in which a bottle, etc. is made from the preform, the areas that are tempered differently are expanded to different degrees. On the one hand, this makes it possible to form walls of varying thickness in different areas of the bottle. On the other hand, selective tempering can be used to counteract the differing expansion of the preform material during the preforming process that arises from the asymmetrical shaping of oval, rectangular or otherwise asymmetrically shaped models to avoid large differences in wall thickness.
A generic device is for example described in U.S. Pat. No. 4,151,249. With this device, preforms are selectively tempered from the outside to create ribs in a bottle. The tempering process can involve either heating or cooling.
DE 19608570 A1 also discloses a generic device for selectively tempering preforms.
A disadvantage manifested by such devices and methods is that the preforms with thick walls of several millimeters can only be unevenly heated through the thick walls. For this reason, substantial stress arises within the material in the mold blowing process.
Wait times to allow the temperature to equalize between the inside and outside of the preform are contrary to high preform throughput rates.
EP521773 B1 discloses the possibility of heating preforms indirectly from the inside by placing an object in the preform and heating it from the outside.
A disadvantage manifested by such devices is that indirectly heating the preform causes the system to become very slow, i.e. heating by using an object that itself has to be heated. The internal body only slowly reacts to changes in the heated supplied from the outside so that the preform is subsequently heated or cooled at a much later time.
When manufacturing bottles at a high throughput rate where brief production stops can arise, this is a major disadvantage since the supply of heat cannot be stopped quickly enough.
Furthermore, DE2852061 discloses a method in which finished bottles are homogenously pre-cooled from the inside.